The exhaust products of internal combustion engines are known health hazards to human beings, animals as well as plant life. The pollutants are, in general, non-burnt hydrocarbons, carbon monoxide, nitrogen oxides, as well as residual amounts of sulfur and sulfurous compounds. Exhaust catalysts have to meet stringent requirements with respect to light-off performance, effectiveness, long-term activity, mechanical stability as well as cost effectiveness in order to be suitable for vehicle application. The pollutants of non-burnt hydrocarbons, carbon monoxides as well as nitrogen oxides have been successfully treated by contact with multifunctional, noble metal catalysts which are capable of converting a high percentage of the pollutants into less harmful products of carbon dioxide, water (steam) and nitrogen. However, the sulfur and sulfurous compounds present in fuels and, in turn, in exhaust product, have been known to poison the noble metals resulting in lessening their catalytic effectiveness and life.
The “catalytic converter” used to convert the harmful pollutants into non-harmful gases, usually consists of three components, that is, the catalytically active metal, the support on to which the active metal is dispersed, and a substrate on to which the support is applied or “washcoated”.
The catalytic metals that are useful to cause effective conversion of harmful pollutants, like carbon monoxide, nitrogen oxides, and non-burnt hydrocarbons under the varying conditions encountered, are noble metals, usually the metals of the platinum group, such as platinum, palladium, rhodium and mixtures thereof. These noble metal catalysts are well known in the art and are more fully described in, for example, DE-05 38 30 318.
The noble metal is typically supported on high surface area inorganic oxides, such as high surface area alumina particles. The high surface area alumina is applied or “washcoated” onto a ceramic or metallic substrate, such as in the form of a honeycomb monolith or wire mesh or the like structure. It is also possible to apply the noble metals onto the support after washcoating the support material onto the monolith.
U.S. Pat. No. 6,335,305 discloses a catalyst that comprises an inorganic oxide support and a noble metal loaded on the support, wherein the support comprises a porous oxide and a composite oxide of the formula (Al2O3)a(CeO2)b(ZrO2)1-b, wherein a is from 0.4 to 2.5 and b is from 0.2 to 0.7.
EP 2 036 606 and EP 2 036 607 describe inorganic oxides comprising aluminum oxide, a metal oxide that does not form a composite oxide with aluminum oxide and at least one additional element selected from rare earth elements and alkaline earth elements useful as a catalyst for treating exhaust gases that is disclosed as having excellent heat resistance.
It is desired to form a porous inorganic composite oxide that exhibits improved heat thermal stability and improved phase stability at elevated temperature.